1. Field of the Invention
The present invention relates to photonic band gap (PBG) materials and method and means of preparation of the same. A definition of the concept of a photonic band gap material is in order, and may be stated as follows. In direct conceptual analogy to the appearance of an electronic band gap in a semiconductor material, which excludes the possibility that electrical carriers can have stationary energy states within the band gap, one can theoretically postulate the appearance of a photonic band gap in a dielectric medium in which the possibility of stationary photonic energy states (i.e., electromagnetic radiation having some discrete wavelength or range of wavelengths) must be rigorously excluded in that band gap. In semiconductors, the electronic band gap results as a consequence of having a periodic atomic structure upon which the quantum mechanical behavior of the electrons in the material must attain eigenstates. By analogy, the photonic band gap results if one has a periodic structure of a dielectric material where the periodicity is of a distance suitable to interact periodically with electromagnetic waves of some characteristic wavelength that may appear in or be impressed upon the material, so as to attain quantum mechanical eigenstates.
In particular, one class of uses of these materials that can be envisioned, but which so far has not been practically demonstrated, is the optical analog to semiconductor behavior, in which a photonic band gap material, or a plurality of such materials acting in concert, can be made to interact with and control light wave propagation in a manner analogous to the way that semiconductor materials can be made to interact with and control the flow of electrically charged particles, i.e., electricity, in both analog and digital applications.
2. Description of the Prior Art
A review of possible applications of these materials is presented by Henry O. Everitt in an article entitled, "Applications of Photonic Band Gap Structures", in Optics and Photonics News, volume 3, number 11, pages 20-23, which was published in November, 1992.
Prior descriptions of photonic band gap materials and methods of preparation of these materials include those of Eli Yablonovitch and co-workers, which have appeared in articles in the scientific journal Applied Physics Letters. These authors have demonstrated that the concept of a photonic band gap material is valid, but have only shown this behavior in the microwave region of the electromagnetic spectrum. They discuss conceptual methods of manufacturing such materials, which rely on the removal of predetermined volumes of material from a solid mass of a uniform material.
These prior inventions have never been applied in commercial practice, but have been described in the scientific literature only, as far as we have Keen able to determine.
We will present an example which demonstrates the superiority of the present invention over the prior art in photonic band gap materials. The major applications of photonic band gap materials are likely to be in the areas of the use and control of electromagnetic radiation in the wavelength range extending from the millimeter or microwave region to the ultraviolet region. The prior descriptions of methods that may be employed to fabricate such materials typically involve the mechanical drilling or machining of holes or cavities of macroscopic dimensions (of the order of millimeters or tenths of millimeters) in solid blocks of a dielectric material, or the concept of using physically directed and orientationally controlled chemical removal such as reactive ion etching to fabricate holes or cavities having dimensions of the order of microns in solid blocks of a dielectric material. These procedures suffer from the disadvantages that they are time consuming, expensive to perform, and require sophisticated and expensive machinery for their practice. Everitt, in the November 1992 publication cited above, states on page 23:
"However, a number of issues must be addressed before the crystals can live up to expectations. Fabrication difficulty increases with increasing band gap frequency. Materials with high real dielectric constants and low loss tangents must be identified for all frequency regions. Theories describing pure and doped PBG crystals must be refined and optimal structures identified. Finally the intolerance of the crystals to imperfections and the lack of post-fabrication gap/defect tunability are practical concerns an experimenter must face.
Nevertheless, given that PBG crystals were first proposed five years ago and demonstrated only last year, researchers are optimistic that these obstacles can be mitigated. For the interested reader, a more in-depth survey of current activity involving PBG crystals and their potential applications may be gleaned from an upcoming issue of JOSA B dedicated to the "Development and Application of Materials Exhibiting Photonic Band Gaps," (February 1993)."
JOSA B refers to the Journal of the Optical Society of America, part B.
Our invention provides a solution to these problems, and is, as far as we can determine, the first to demonstrate photonic band gap behavior in the near infrared and visible portions of the electromagnetic spectrum.
In addition, we have discovered other new details of construction, which, when taken together, permit our invention to achieve results which the previously disclosed technologies are not capable of achieving. These discoveries will be stated explicitly in the discussion of the invention.
Yablonovitch et. al. teaches a method and means for the construction of a photonic band gap material in Applied Physics Letters volume 67, number 17, pages 2295 to 2298, published on Oct. 21, 1991, in which it is stated, on page 2296:
"A slab of material is covered by a mask containing a triangular array of holes. Three drilling operations are conducted through each hole, 35.26.degree. off normal incidence and spread out 120.degree. on the azimuth. The resulting crisscross of holes below the surface of the slab provides a fully three-dimensional periodic fcc structure, with WS unit cells . . . The drilling can be done with a real drill bit for microwave work, or by reactive ion etching to create an fcc structure at optical wavelengths. We have fabricated such crystals in the microwave region by direct drilling into a commercial, low-loss, dielectric material, Emerson and Cumming Stycast-12. Its microwave refractive index, n.about.3.6, is meant to correspond to the that of the common semiconductors, Si, GaAs, etc. By simply scaling down the dimensions, this structure can be employed equally well at optical wavelengths."
In an article entitled "Photonic Band Gaps", published in the August, 1992 issue of Physics World, pages 37 to 42, Philip St. J. Russell states, (page 37),
"Despite the difficulties of designing and constructing the right kind of structure, and of detecting what happens, Yablonovitch's team have observed a PBG at microwave frequencies in a specially drilled dielectric material (microwave refractive index of 3.6) with a face-centered-cubic lattice. This "dielectric crystal" was produced by drilling evenly spaced (8 mm pitch) sets of holes at three carefully chosen angles. To obtain a band gap at optical frequencies requires a very much smaller lattice spacing (.about.400 nm for 1.5 .mu.m light in GaAs) and is much more challenging to produce. Although techniques involving reactive ion beam etching are being actively developed, no success has yet been reported."
Since Yablonovitch's device is fabricated by the removal of a selected portion of the slab of the starting material, it is clear that the present invention differs significantly and qualitatively from the device of Yablonovitch with regard to its underlying principles of fabrication. Each of the prescriptions in Yablonovitch's papers is based specifically on this geometry and method of construction, in which the structure is fabricated from a solid slab of the starting material from which material is removed.
A second type of structure having holes or pores has also been fabricated by an etching process applied to single crystal silicon. Recently, L. T. Canham at the Royal Signals and Radar Establishment (RSRE) in the United Kingdom reported the ability of anodized crystalline silicon ("porous" silicon) wafers to emit light in the visible under illumination, with no electrical contacts attached (Applied Physics Letters 57, 1046 (1990)). This result, which has been confirmed by other groups, is not well understood but is reproducible. It appears to depend on the fabrication of "quantum wires", having diameters measured in nanometers and lengths of some microns. According to RSRE, electrochemical and chemical dissolution are used to etch a thin layer of free-standing wires in the surface of bulk silicon wafers. While this etching process is not yet understood, it produces these wires rather than merely removing some thickness of silicon in a uniform manner. The dimensions of the wires may be related to the wavelength of the light which is emitted. Under the action of a blue (488 nm) laser, in excess of one percent of the incident light can be emitted in the green, while under green laser light emission in the red is observed. Both of these findings suggest that relatively high absorption efficiency should be possible. Some of these results have appeared in the literature.
In contradistinction to Yablonovitch and to Canham, we have discovered that it is possible to fabricate a photonic band gap material by impregnating the pores or voids contained within the volume of a specially prepared reticulated mesh, which may be made of a material with a high melting temperature such as a metal, with liquid material which melts at a temperature lower than the melting temperature of the reticulated mesh and which solidifies upon cooling. The reticulated mesh is then dissolved by simple chemical action in a liquid bath, leaving behind a solid reticulated structure composed of the solidified liquid material. In particular, the liquid material may be caused to solidify into an ordered solid such as a single crystal by the imposition of either or both a thermal gradient or a seed single crystal of the same or a closely related material. Rather than being a subtractive process, such as those of Yablonovitch or Canham, in which material is removed from a single mass, our process is an additive process in which a structure is produced by solidifying or adding material to form a structure. A template can be employed which may or may not be removed, as may be required, after the additive process is carried out.
Many embodiments of the present invention can be envisioned. In one instance, the material can be made as a reticulated single crystal of a material having a high dielectric constant, such as sapphire. In another instance, the material could be made as a polycrystalline reticulated solid of a material having a high dielectric constant. Other embodiments could be made from a reticulated single crystal of doped material having high dielectric constant, such as ruby (sapphire doped with chromium in 3+ oxidation state) or titanium-doped sapphire, both of which are in themselves laser materials. Yet other embodiments could be made from reticulated doped glass, such as neodymium doped glass, which is also a laser material. Yet further embodiments of the invention can be recognized in which a second material, having a second, different dielectric constant and different optical behavior, is introduced into the voids of the reticulated solid material. Still further embodiments can be suggested which may consist of reticulated bodies of material having high dielectric constant with pore sizes of dimensions significantly larger or smaller than approximately 10 microns, for example in the range of approximately 200 microns to approximately 1 micron or perhaps less, if the reticulated metal structures can be made, for example by the freezing of electrohydrodynamically generated droplets or mist. Yet further embodiments can be pointed out, as regards the means of impregnation of the fluid into the reticulated template solid, which can include capillary action, as when a liquid such as water is blotted up by tissue paper, or which can include a pressure differential, as when a gas is allowed to fill a volume, either by raising the pressure at the entry or diminishing the pressure at the exit, or a combination of both.
In addition, photonic band gap materials and devices constructed according to the prescriptions given in the present invention may be used in optical equipment and machines of more advanced design than those manufactured previously.